1. Field of the Invention
The invention relates to a thin film forming apparatus for forming a thin film on a substrate such as a glass substrate, Si-semiconductor, and the like, for instance.
2. Description of the Related Art
As a technique for forming a thin film on a surface of a glass substrate, a spray pyrolysis deposition technique is known. In the spray pyrolysis deposition technique, by spraying a material solution containing a thin film forming material on one surface portion of a heated substrate, a solid phase is deposited from a liquid phase on the heated substrate, and a thin film is formed on one surface portion of the substrate. (For instance, refer to Kaneko Shoji “An outline of a spray pyrolysis deposition (SPD) technique and its application to tin oxide transparent conducting film formation”, Function & Materials, CMC Publication Co. Ltd., (Vol. 20, No. 3, March 2000): p. 5-7.) In detail, in the spray pyrolysis deposition technique, a material solution is prepared by dissolving a thin film material in an organic solvent. When this material solution is mixed with a compressed gas and a jet of the mixture is then emitted to a heated plate-like substrate, misty liquid droplets of the material solution are supplied to a surface of a plate-like substrate. The liquid droplets of the material solution are heated on the surface of the plate-like substrate, and an organic solvent as liquid phase evaporates and gasifies. As a result, a thin film material as solid phase precipitates, and the thin film material precipitated grows with deposited on the surface of the plate-like substrate, thereby forming a thin film. As compared with other chemical thin film forming processes such as an atmospheric chemical vapor deposition (CVD) process, the spray pyrolysis deposition has advantages of being capable of forming a film even at a low heating temperature of the substrate, a high rate of film formation and less frequency of maintenance due to simple construction of the apparatus.
In a thin film forming apparatus of conventional art for forming a thin film by using a spray pyrolysis deposition technique, a thin film is formed on one surface portion of a glass substrate heated to a prescribed temperature, by spraying from a nozzle a material solution containing a thin film forming material, together with compressed air. In the thin film forming apparatus, a thin film having a prescribed thickness is formed, by repeating a plurality of times the spraying of the material solution containing a thin film forming material, thereby laminating a thin film. (For instance, see Japanese Unexamined Patent Publications JP-A 10-130097 (1998) AND JP-A-2001-26885.)
In a thin film forming apparatus of a conventional art, a thin film can be formed relatively easily on a surface of a glass substrate, by heating the glass substrate to a prescribed temperature and by spraying a material solution containing a thin film forming material. However the thin film formation speed is not always constant because of, for instance, temperature changes of a glass substrate, changes with time of a nozzle for spraying a material solution, changes in concentration of a material solution and changes in atmosphere of a glass substrate. This poses a problem that it is difficult to form a thin film of a prescribed thickness. The changes in atmosphere of the glass substrate include changes in air current of the glass substrate atmosphere, changes in temperature of the glass substrate atmosphere and changes in humidity of the glass substrate atmosphere.
FIG. 20 is a perspective side view showing schematically the constitution of a conventional thin film forming apparatus 200. The thin film forming apparatus 200 comprises spraying means 203, spraying control means 204, heating means 205, and exhausting means 206. Spraying means 203 emits a jet of a material solution containing a thin film material and an organic solvent to a plate-like substrate 201 and spraying liquid droplets 202 of the material solution. Spraying control means 204 for controls the spraying quantity of the material solution. Heating means 205 heats the plate-like substrate 201. Exhausting means 206 exhausts to the outside an organic solvent vapor evaporating from the material solution by heating. Furthermore, the spraying means 203 comprises material solution reservoir means 207, gas reservoir means 208, and a spray nozzle 209, Material solution reservoir means 207 reserves a material solution. Gas reservoir means 208 reserves a carrier gas to be mixed in the material solution. A spray nozzle 209 mixes the material solution and carrier gas and sprays the mixture on a surface of the plate-like substrate 201. As a carrier gas, compressed air and the like are used. The spraying control means 204 comprises a solenoid valve 210 and valve control means 211. A solenoid valve 210 controls the ON and OFF of the spray nozzle 209. Valve control means 211 controls the opening and closing of the spray nozzle 209 by the solenoid valve 210. The heating means 205 comprises a heater 212 and a stage 213. The heater 212 heats the plate-like substrate 201 The stage 213 holds the plate-like substrate 201 and the heater 212, The stage 213 on which the plate-like substrate 201 and the heater 212 are held is carried to just below the spray nozzle 209 by a carrying means such as a belt conveyer (not shown).
According to the thin film forming apparatus 200, first a material solution from the material solution reservoir means 207 and also a compressed gas from the gas reservoir means 208 are supplied to the spray nozzle 209 respectively. In the spray nozzle 209, the material solution and the compressed gas are mixed, a jet of the mixture is emitted to a surface 201a of a plate-like substrate 201 held by the stage 213, thereby supplying liquid droplets 202 of the material solution by spraying. Since the plate-like substrate 201 is heated to a prescribed temperature by the heater 212, an organic solvent evaporates and vaporizes from the liquid droplets 202 of the sprayed material liquid to allow the thin film forming material to precipitate, thereby forming a thin film on the surface 201a of the plate-like substrate 201. The organic solvent vapor is exhausted to the outside by the exhausting means 206. By repeating this spraying operation a plurality of times, a thin film having a desired thickness can be formed.
As described above, in the thin film forming apparatus 200 based on a spray pyrolysis deposition technique, the spray nozzle 209 is arranged approximately just above the central part of the plate-like substrate 201 to be carried from the outside and be held in a prescribed position.
Meanwhile, when a solution is sprayed from a nozzle to the central part of a flat plane-like substance, the spraying quantity per unit area of the solution becomes large at the central part of the plate-like substance and small at the ends, because of the constructive characteristics of the nozzle.
Accordingly, when a thin film is to be formed on a surface 201a of a plate-like substrate by using a conventional thin film forming apparatus 200, a thin film raised in a direction perpendicular to the surface 201a is formed at the central part of the plate-like substrate 201 because of the position of the spray nozzle 209 and the constructive characteristics, and at the ends of the plate-like substrate 201, a thin film whose thickness is smaller than that at the central part. It is very difficult to form a thin film having a uniform thickness on the entire part of the surface 201a of the plate-like substrate 201 by using the thin film forming apparatus 200. The thin film forming apparatus 200 is not suitable for the formation of an antireflection film requiring an especially uniform thin film.
A method to adjust the thickness of a thin film by moving the plate-like substrate 201 to the position so that the end is just below the spray nozzle 209 after a thin film is formed centering around the central part of a plate-like substrate 201 and by spraying the material solution again can be considered. However, such a method will cause an increase in the number of steps and an increase in material costs, and yet the uniformity of a thin film to be obtained is not fully satisfactory.